ML20077E779

Submits Rept on Remeasurement of MITR-II Temp Coefficient & Analysis of Results.Core Temp Coefficient of Reactivity Negative in Normal Operating Range & Existence of Positive Uniform Temp Coefficient Not Safety Hazard

ML20077E779
Person / Time
Site:	MIT Nuclear Research Reactor
Issue date:	07/22/1983
From:	Lisa Clark MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE
To:	Allan J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
NUDOCS 8307280441
Download: ML20077E779 (5)

Text

{{#Wiki_filter:I 1 4 +*(.3 - 9' NUCLEAR REACTOR LABORATORY g ( % y%# AN INTERDEPARTMENTAL CENTER OF MASSACHUSETTS INSTITUTE OF TECHNOLOGY O.K. HARLING 138 Albany Street Cambridge, Mass. 02139 L. CLARK, JR. Director (617) 253-4 2 0 2 Director of Reactor Operations July 22, 1983 Mr. James M. Allan, Acting Administrator Region #1 U.S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406

Subject:

Remeasurement of Temperature Coefficient, Facility Operating License #R-37, Docket 50-20

Dear Mr. Allan:

The purpose of this letter is to report on the remeasurement of the MITR-II temperature coefficient and to provide our analysis of the results. The temperature coefficient was remeasured on January 17, 1983. The results have been under review and are now reported herein because they differ somewhat from the measurement reported and discussed in MIT's letter to USNRC Region #1 dated March 23, 1977. The results differ principally at the low end of the temperature range, which was extended down to 8*C as compared to a minimum temperature of 15*C for the 1977 data. The uniform temperature coefficient was found to be positive below 21*C in contrast to the earlier data. Above that temperature, the coefficient was measured to be negative on both occasions. For clarity the following *.emperature and void coefficients, C and C, T y are defined. CT (Unif.) Unli,rm (or combined C (H O) + CT (D 0)) temperature 2 7 2 coefitcient of reactivity C7 (D 0) D 0 Leaperature coefficient of reactivity 2 CT (H O) H O temperature coefficient of reactivity 2 2 T (Cere) Core temperature coefficient of reactivity C CT (H O refl.) H O reflector temperature coefficient of reactivity 2 2 CT (Shim Bank) Reactivity effect due to shim bank pasition change with temperature 8307290441 830722 PDR ADOCK 05000020 P PDR hp y

page 2 Cy (D 0) D 0 void coefficient of reactivity 2 2 Cy (H O refl.) H O reflector void coefficient of reactivity 2 2 Cy (Core) H O core void coefficient of reactivity 2 The recent measurements, as well as those evaluated in the 1977 letter, involved CT (Unif.), where the reactivity is measured at low power as the reactor temperature, relatively uniform throughout the core,H O and D 0 re-2 2 flectors, is slowly raised by means of pump heat. The data evaluated in the 1977 letter varied from 15*C to 50*C, and it was stated that CT (Unif.) "could be slightly negative or positive, depending on the curvature of the data in the region 10*-15*C". It was concluded that the more important CT (Cora) would be " negative at all temperaturesabove 10*C, i.e. over the entire operating range for the reactor". The recent measurements show that CT (Unif.) is clearly positive in the range 8-21*C, but this does not alter the conclusion that CT (Core) is " negative at all temperaturesabove 10*C", as is shown below. The purpose of the new measurement was to obtain data over as wide a range as possible for use in a computer program being developed for control of reactor power levels. It was desired to obtain data to the lowest feasible tcmperature, i.e. 8*C, which is only slightly warmer than the 4 C freezing paint of D 0. The reactor temperature normally does not drop below 15*C, and 2 an alarm alerts the operator if the secondary should drop as low as 10 C. A plot of the recent measurements is shown in the attached figure. The data are plotted so that a zero reactivity exists at 21*C. C7 (Unif.) is positive below that temperature, and the reactivity loss to 8 C is 52 mS. CT (Core) is related to the other temperature coefficients by the follow-ing expression: CT (Unif.) =CT (Core) + CT (H O refl.) + CT (D 0) + CT (shim bank) 2 2 CT (Core) must be negative if the three most righthand terms are i l sufficiently positive that their combination exceeds the positive value of CT (Unif.). As mentioned above the reactivity gain on increasing the uniform temperature from 8* to 21"C is 52 mS. 3 The void coefficient, Cy (H2O refl.), was measured to be + 0.28 mS/cm during MITR-II start-up testing (l). The measurement was made in one of the The radial H O reflector averages about 2 cm thick, core housing corner holes. 2 and the bottom H O reflector about 6 cm thick. A temperature increase from 2 8*C to 21*C results in a 0.186% density decrease, equivalent roughly to a 21.9 mS reactivity increase, assuming the top H O reflector effectiveness to 2 be equal to that of the 6 cm bottom reflector. Ref. (1): "MITR-II Start-up Report", MITR Staff, Report No. MITNE-198 (February 1977)

Page 3 Also during the MITR-II start-up testing the D 0 temperature coefficient, 2 T (D 0), was measured to be +1.6 mS/*C in the range 21*-28"C. Assuming a C 2 no less positive effect at the lower temperature, a reactivity gain of 20.8 mS is to be expected on increasing the D 0 temperature from 8' to 21*C. 2 The third coefficient, CT (shim bank), results from motion of the shim bank with respect is the fuel due to temperature changes, and is the result of different coefficients of expansion for different structural materials, that for Al being more than double the coefficient for stainless steel. Fuel is supported in the Al core tank, which will expand faster than the stain-less steel drive shafts from which the shim blades are supported, the effect being a slight shim bank withdrawal upon uniform heating. At the peak shim bank reactivity (~850 mS/in), the reactivity increase is about 0.785 mS/*C. This amounts to 10.2 mS over the range 8-21*C Thus, the combined positive reactivity effects of the H O reflector, 2 D 0 reflector, and shim bank motion total +52.9 mS. Because of the un-2 certainties associated with the above caluclations, they should be regarded as qualitative only, even though the sum is fortuitously close to the total measured effect. Nevertheless they do explain essentially all of the positive reactivity and do support the conclusion that CT (Core) is negative rather than positive at all temperatures above 8*C. The relative expansions of fuel and moderator as temperatures increase also support a negative CT (Core). At 7*C, the volume coefficients of expan-sion for aluminum and water are about equal (0.67 x 10-4 *C). Above this / temperature the coefficient for water increases much more rapidly with tem-perature than does that for aluminum (a factor of ten larger by about 32*C), so that the relative expansions of moderator and fuel in this undermoderated core will lead to further undermoderation as temperature increases, resulting in a negative CT (Core). Measurements in the core further support a negative C (Core). During T start-up tests (l), negative void coefficients, Cy (Core), were measured in all three fuel rings. This is expected since the core is undermoderated. It is CT (core) and Cy (Core), not the other temperature coefficients discussed l above, that determine the characteristics of any power transient. It might be added that, in any event, a positive reactivity addition of 52 mS on heating from 8"C to 21*C would only result in a steady rising reactor period of 220 seconds. This is easily controlled, as was proven during the test. A further rise of 35'C would entirely eliminate the positive reactivity. Reactor operation in this low temperature range is carefully and routinely avoided, in order that there will be no danger of freezing the D 0 (at 4*C). 2 An alarm activates at 10*C, as indicated above. Also operation in this tem-perature range is only possible at low reactor powers, because any significant heating raises reactor temperatures to the ranges where the coefficient rapidly becomes increasingly negative.

page 4 i It is concluded that the core temperature coefficient of reactivity is negative in the normal operating range and that the existence of a positive uniform temperature coefficient in the 8-21*C range does not in any way constitute a safety hazard. This letter has been reviewed and approved by the Subcommittee of the MIT Reactor Safeguards Committee. Sincerely, N Yk Lincoln Clark, Jr. Director of Reactor Operations LC/sbs i cc: MITRSC USNRC-DMB USNRC-0MIPC I ._y.,_ -, -. _ __ _ _ - _ _ --,..- _ - _.,,._..._...,.....,_-,,.,..,,,,._,_m_.,,, ,._-.,v_- ..-..--o __._y,,

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